Has the moment of truth arrived? A traumatised Paris will be in lockdown this week as more than 140 world leaders, including Barack Obama and President Xi Jinping of China, fly into the city for the start of two weeks of climate change negotiations which the UN hopes will lead to a historic new global deal to reduce carbon emissions.

Has the moment of truth arrived? A traumatised Paris will be in lockdown this week as more than 140 world leaders, including Barack Obama and President Xi Jinping of China, fly into the city for the start of two weeks of climate change negotiations which the UN hopes will lead to a historic new global deal to reduce carbon emissions.

No demonstrations will be allowed in France but hundreds of thousands of people are due to march today in Britain, Australia, the US, South Africa, Brazil and Europe.

An unprecedented security operation following the terrorist attacks in Paris will see 2,800 police guarding the 40,000 delegates and diplomats and 6,000 journalists who will pack the conference venue at Le Bourget airport.

A further 8,000 police will guard French borders. "Mobile units, riot squads and gendarmerie units will all be called on at an unprecedented level," said the interior minister, Bernard Cazeneuve.

The mayor of Paris, Anne Hidalgo, urged Parisians not to use their cars today when much of the city will be virtually impassable as world leaders host bilateral meetings in hotels and embassies.

The summit, which hopes to secure a deal to hold global warming to a 2C rise, opens to a warning that climate change destabilises countries and allows terrorism and conflict to flourish.

"We are quite certain that the impact of climate change will be to destabilise countries," Jim Yong Kim, the World Bank president, told the Observer. The bank claimed that terrorist movements such as Boko Haram in Nigeria and al-Shabaab in Somalia have taken root because of droughts and growing competition for scarce resources fuelled by climate change.

Prince Charles will give the keynote speech at the summit of 195 countries and will warn that there is little time to avoid climate catastrophe.

The state of emergency in France, which is due to last three months, has resulted in hundreds of events around the conference being cancelled.

However, some demonstrators are expected to defy police in the name of free speech.

Nine French climate activists have been placed under house arrest, accused of flouting a ban on organising protests. Naomi Klein, the climate change campaigner, accused authorities of "a gross abuse of power that risks turning the summit into a farce. The French government, under cover of anti-terrorism laws [is] shamefully banning peaceful demonstrations and using emergency powers to pre-emptively detain activists".

Greenpeace said people would make themselves heard, whatever the obstacles. Jean-Francois Julliard, executive director of its French arm, said: "March or no march, in Paris thousands of people will use their collective imagination to project their voices into the UN climate talks."

UN bodies, international charities and academics have all urged governments to act boldly. "Climate change is the defining issue for the 21st Century. We estimate that it is already causing tens of thousands of deaths every year - from shifting patterns of disease, from extreme weather events, and from the degradation of air quality, food and water supplies," said a spokesman for the World Health Organisation.

Mark Goldring, the UK chief executive of Oxfam, warned that developing countries' economies could be wrecked by climate change - they faced losing €1.6tn annually by the middle of the century if temperatures rise by 3C. "Leaders need to put aside their self-interest and do what is best for the world," he said.

FIVE REASONS TO BE CHEERFUL:

1. The world really wants a strong deal and this time will get it. There is a universal will to limit emissions, governments understand the science and know that doing nothing is no longer a political or moral option. Evidence of climate change has grown since the Copenhagen summit in 2009, and 2015 has already been declared the hottest year on record.

2. A green economy makes financial sense. A bold new international deal committing all countries to reduce emissions is in everyone's long-term economic interests. It will signal to business that governments are legally committed to reducing emissions and will give the private sector and banks the long-term confidence they need to invest in renewable energies and conservation.

3. Nations are ready to commit to real change. Most countries have already stated their intentions. More than 180 countries, representing 90pc of global emissions, have submitted plans to cut emissions. This is the first time since negotiations started 20 years ago that nearly all the world's nations have committed to being part of the solution.

4. The text is manageable. The chances of diplomatic success are much higher than in Copenhagen in 2009, which ended in diplomatic chaos. The text that negotiators will haggle over in Paris is shorter and more focused and many difficult decisions have already been made.

5. We're all in it together. The recent Paris terrorist attacks will galvanise world leaders to make a global statement of solidarity. No country will want to be identified as the one that stopped a deal.

FIVE REASONS TO BE FEARFUL:

1. Countries may not make the necessary compromises. Presidents, prime ministers and heads of state will make short statements about the need to act, after which negotiators and politicians will have just days to reach agreement. Given that it has taken 20 years to reach this point, there is little chance that the gaps between countries can be closed so fast. So the only way a deal can be reached is if the UN and France, as the hosts, bludgeon through a least-worst agreement over the heads of the many.

2. The fat lady has already sung. The cuts that 180 countries have said they are prepared to make up to 2030 will hold global temperatures to a 2.7C rise. The maximum rise, if catastrophic warming by the end of the century is to be averted, is 2C. More than 100 countries have said they want the UN to set the more ambitious target of 1.5C. For them, anything that does not guarantee this will be a failure.

3. Who will bear the biggest burden? Countries are still split on issues like reducing emissions, finance and technology. The most important hurdle could be over whether industrialised countries should be obliged to cut more than developing countries.

4. Where's the money? Many of the ambitious plans to cut emissions depend on €1tn being made available to invest in renewable energy, farming and forestry. Developing countries will not roll over without financial guarantees.

5. There is a will to tackle climate change but not at any price. There is still a deep distrust of the way the US and others have avoided having to change their lifestyles but have bullied poor countries to shoulder the burden of cuts.